Calculation of Estimated Ultimate Recovery and Recovery Factors of Shale-Gas Wells Using a Probabilistic Model of Original Gas in Place

Estimation of original gas in place (OGIP) in a rock volume provides an upper limit for the expected ultimate recovery. Calculation of OGIP for a drainage region may introduce significant errors when dependent on discrete values; the deterministic OGIP may be either overestimated (positively biased) or underestimated (negatively biased). For example, parameters such as porosity, water saturation, and adsorbed-gas density may vary spatially, which must be accounted for to obtain realistic OGIP estimations. Our objective was to develop a more-accurate OGIP model and use it to probabilistically asses OGIP, estimated ultimate recovery (EUR), and recovery factor (RF) for shale-gas reservoirs such as the Marcellus Shale. The conventional OGIP model was modified to include recent developments in shale geology and gas adsorption. Corrections are made by considering pore space occupied by adsorbed-gas phase, porosity, and water saturation in both matrix and fracture systems to obtain improved OGIP estimations. This change was assessed in the context of both the Langmuir (1918) and Brunauer-Emmet-Teller (BET) -isotherm (Brunauer et al. 1938) adsorption models. In addition, a 25-year EUR response-surface model was created using a comprehensive semianalytical model by capturing multiple gas-transport mechanisms such as gas desorption, slippage, diffusion, and non-Darcy flow. The OGIP and EUR models were coupled during Monte Carlo simulation to produce a more-accurate probability distribution for RF. Using the prospective land area in the entire Marcellus Shale, the total P50 OGIP, P50 EUR, and P50 RF were determined as 1,313 Tcf, 492 Tcf, and 38%, respectively.